Have you tried driving it around for 50-60 miles or so in Sport? In Auto, it can be a little lazy, unless you give it half+ throttle and then it will downshift. I drive around in Auto most of the time, and it shifts into 8th gear by 40mph. It seems perfectly content at 1500rpm or so (which might be considered lugging the engine), but I've gotten used to it.
If you don't want it upshifting into a really high gear, put it in Sport and drive it like that... no more lugging.
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